Positive active material for rechargeable lithium battery, method of preparing the same, and rechargeable lithium battery comprising the same

ABSTRACT

The present invention relates to a positive active material for a rechargeable lithium battery, a method of preparing the same, and a rechargeable lithium battery comprising the same. The positive active material includes a lithium/nickel-based compound wherein primary particles having an average particle diameter ranging from 1 μm to 4 μm are agglomerated to form secondary particles. The positive active material of the present invention has excellent electrochemical performance and outstanding inhibition to swelling at high temperatures.

CROSS REFERENCE TO RELATED APPLICATION

This application claims priority to and the benefit of Korean PatentApplication No. 10-2004-0007896, filed on Feb. 6, 2004, which is herebyincorporated by reference.

FIELD OF THE INVENTION

The present invention relates to a positive active material for arechargeable lithium battery, a method of preparing the same, and arechargeable lithium battery comprising the same. More specifically, theinvention relates to a positive active material for a rechargeablelithium battery that has excellent electrochemical performance andoutstanding inhibition of swelling at high temperatures, a method ofpreparing the same, and a rechargeable lithium battery comprising thesame.

BACKGROUND OF THE INVENTION

The use of portable electronic equipment has increased as the deviceshave become lighter and smaller, due to recent developments in theelectronics industry. Accordingly, research on batteries as a powersource for this portable electronic equipment with a higher energydensity has also increased.

A rechargeable lithium battery is fabricated by filling an organicelectrolytic solution or polymer electrolyte between positive andnegative electrodes. The electrodes reversibly intercalate anddeintercalate lithium ions, and produce electrical energy throughoxidation and reduction reactions during the intercalation anddeintercalation of the lithium ions.

Lithium has commonly been used as the negative active material forrechargeable lithium batteries. Carbon-based materials such ascrystalline and amorphous carbon have been replacing lithium metalbecause lithium causes short circuits in the batteries as a result ofthe formation of dendrites, and can also bring about explosions.

The positive active material is known to play the most important role insafety and performance of a rechargeable lithium battery. Because ofthis, chalcogenide compounds or complex oxide materials such as LiCoO₂,LiMn₂O₄, LiNiO₂, LiNi_(l-x)Co_(x)O₂ (0<x<1), LiMnO₂, and so on are beingstudied. Cobalt-based materials, including LiCoO₂ are the most widelyexamined as a positive active material due to high energy density(theoretical capacity of 274 mAh/g with LiCoO₂) and excellent cycle lifecharacteristics (capacity retention).

However, due to its structural instability, only about 50% of thetheoretical capacity of LiCoO₂, which is approximately 140 mAh/g at acharge voltage of 4.2V, is practically obtained that is to say, thenon-utilized amount of Li in Li_(x)CoO₂ is over 50% (x>0.5). In order toobtain more than 50% of the theoretical capacity of LiCoO₂, the chargevoltage must be increased to over 4.2V. But the potentially usable xvalue, which represents the amount of Li in Li_(x)CoO₂, decreases tounder 0.5 when the voltage is increased, and a consequential structuralinstability due to a phase transition from a hexagonal to a monoclinicalstructure also sharply decreases its capacity retention.

As a result, the search continues for a positive active material for arechargeable lithium battery that has a high energy density at a highcharge voltage of greater than 4.2V, and excellent cycle life. Forexample, LiNi_(x)Co_(l-x)O₂ (0<x<1), LiNi_(x)Mn_(l-x)O₂ (0<x<1),Li(Ni_(x)Co_(l-2x)Mn_(x))O₂ (0<x<1), LiCoO₂, and LiNiO₂ derivativeswhere elements such as Ni, Co, Mn, and so on are modified (Solid StateIonics, 57,311 (1992), J. Power. Sources, 43-44, 595 (1993), JapaneseLaid-Open Patent Pyung 8-213015 (Sony Company (1996)), U.S. Pat. No.5,993,998 (Japan Storage Battery) (1997)) are being examined. However,these positive active materials have a disadvantage of poor inhibitionwith regard to swelling at high temperatures.

SUMMARY OF THE INVENTION

An aspect of the present invention provides a positive active materialfor a rechargeable lithium battery that features outstandingelectrochemical performance and excellent inhibition of swelling at hightemperatures.

The present invention also provides a method of preparing a positiveactive material for a rechargeable lithium battery.

In addition, the present invention also provides a rechargeable lithiumbattery comprising the aforementioned positive active material.

In order to accomplish these aspects, the present invention provides apositive active material for a rechargeable lithium battery that is alithium/nickel-based material, wherein primary particles having anaverage diameter of 1 μm to 4 μm are agglomerated to form secondaryparticles.

The present invention provides a method to prepare a positive activematerial for a rechargeable lithium battery comprising the secondaryparticles composed of the primary particles whose average particlediameter is 1 μm to 4 μm. The processes of this preparation are asfollows: preparing an oxide material by heat-treating a hydroxidecompound that includes nickel and cobalt, nickel, cobalt, and a metal(such as Al, Cr, Fe, Mg, Sr, V, and rare earth element), nickel, cobalt,and manganese, or nickel, cobalt, manganese, and a metal (such as Al,Cr, Fe, Mg, Sr, V, and rare earth elements); mixing the oxide materialwith a lithium-containing compound and performing a first heat treatmentto the resulting mixture; and performing a second heat treatment to thefirst heat-treated mixture. The present invention provides arechargeable lithium battery with a positive electrode that includes thepositive active material, a negative electrode that includes a negativeactive material, and an electrolyte. The negative active material mayinclude lithium metal, a lithium-containing alloy, a material which iscapable of reversibly intercalating and deintercalating lithium ions, ora material that can form a lithium-containing compound by reversiblyreacting with lithium.

BRIEF DESCRIPTION OF THE DRAWING

A more complete appreciation of the invention and its advantages willbecome readily apparent as the invention becomes better understood byreference to the following detailed description when considered inconjunction with the accompanying drawings.

FIG. 1 shows a schematic view of a rechargeable lithium battery of thepresent invention.

FIG. 2 is a graph showing the amount of carbon content according to themolar ratio of Li:(Ni+Co+Mn) in positive active materials according toExamples 1 to 5 of the present invention.

FIGS. 3A to 3E are scanning electron microscope (SEM) photographs ofpositive active materials according to Comparative Example 1 andExamples 1 to 4.

FIG. 4 is a graph showing discharge capacity of rechargeable batteriesaccording to Examples 1 to 5 and Comparative Examples 1 and 2.

DETAILED DESCRIPTION OF THE INVENTION

A positive active material for a rechargeable lithium battery of thepresent invention is a lithium/nickel-based compound with secondaryparticles composed of primary particles whose average diameter is 1 μmto 4 μm. The average diameter of the secondary particles comprising theprimary particles is preferably 5 μm to 20 μm, and more preferably 7 μmto 14 μm. When the average diameter of the secondary particles is under5 μm, some side reactions can occur in a rechargeable lithium batterycomprising the same. However, when the average diameter of the secondaryparticles is over 20 μm, the density of positive electrode mixturedecreases and the secondary particles are easy to break, which resultsin the decomposition of the electrolyte and the generation of gas.

According to the present invention, carbon may be present on the surfaceof the positive active material, where the amount of the carbon ispreferably less than 0.1 wt % of the total weight of the positive activematerial, more preferably between 0.05 to 0.07 wt %, and even morepreferably between 0.05 to 0.06 wt %.

The carbon present on the surface of the positive active material is animpurity introduced in the preparation process. Lithium carbonate isformed on the surface through a reaction between CO₂ and unreactedlithium in the positive active material during a calcinating process.When the amount of the carbon increases, the swelling by the lithiumcarbonate also increases. When the amount of the carbon is over 0.1 wt %of the total weight of the positive active material, the inhibition onthe swelling is not improved at high temperatures.

When the average diameter of the primary particles of the positiveactive material in the present invention is under 1 μm, the amount ofthe carbon increases during the fabrication of the positive electrode,which results in a reduction of the swelling inhibition. However, theuse of the primary particles whose average diameter is over 4 μm cancause problems such as decreased capacity.

The X-ray diffraction intensity ratio I(003)/I(104) of thelithium/nickel-based compound has an X-ray diffraction pattern usingCuKα of 1.04 to 1.15, and its full width at half maximum ranges from0.14 to 0.16. When the full width at half maximum is under 0.14, thecrystallinity of the positive active material is high and it can preventthe intercalating of Li ions in the positive electrode. When the fullwidth at half maximum is over 0.16, the crystallinity of the positiveactive material is low and it can prevent the positioning of Li ions inthe positive electrode. Therefore, the use of the positive activematerial with a full width at half maximum that is out of the range of0.14 to 0.16 can cause problems such as decreased capacity.

The lithium/nickel-based compound is represented as one of the followingchemical formulas (1) to (5):Li_(x)Ni_(l-y)Co_(y)O_(2-z)A_(z)   (1)Li_(x)Ni_(l-y-z)Co_(y)M_(z)A₂   (2)Li_(x)Ni_(l-y-z)Co_(y)Mn_(z)O_(2-α)A_(α)  (3)Li_(x)Ni_(l-y-z)Co_(y)Mn_(z)O_(2-α)A_(α)  (4)Li_(x)Ni_(l-y-z)Co_(y)Mn_(z)M_(w)O_(2-α)A_(α)  (5)where x, y, z, w, and a are given by 0.94≦x≦1.1, 0≦y≦0.5, 0≦z≦0.5,0≦w≦0.5, 0≦α≦2, M is at least one element selected from the groupconsisting of Al, Cr, Fe, Mg, Sr, V, and rare earth elements, and A isat least one element selected from the group consisting of O, F, S, andP.

A positive active material of the present invention is prepared bymaking an oxide material through the heat treatment of a hydroxidecompound that includes nickel and cobalt, or nickel, cobalt, and a metalsuch as Al, Cr, Fe, Mg, Sr, V, and rare earth element, or nickel,cobalt, and manganese, or nickel, cobalt, manganese, and a metal such asAl, Cr, Fe, Mg, Sr, V, and rare earth elements. Next, the oxide materialis mixed with a lithium-containing compound and a first heat treatmentis performed on the resulting mixture, and then a second heat treatmentis performed on the first heat-treated mixture.

A lithium/nickel-based positive active material has conventionally beenprepared by mixing a hydroxide compound with a lithium-containingcompound and heat-treating the mixture rather than heat-treating thehydroxide compound alone. This method results in primary particles withan average diameter that does not grow to 1 μm, and a significant amountof carbon remains on the surface, which causes swelling at hightemperatures.

The present invention improves upon the conventional method as itincreases the average diameter of the primary particles up to 1 μm anddecreases the amount of the carbon on the surface to under 0.1 wt % ofthe total weight. It achieves this by heating the hydroxide compound toform the oxide compound, and then reacting it with a lithium compoundand subjecting the resulting mixture to two heat treatments.

The oxide compound is formed by heating the hydroxide compoundpreferably for 5 to 10 hours at 500 to 1200° C., and more preferably for5 to 10 hours at 700 to 1000° C. At heating temperatures under 500° C.or a treatment time less than 5 hours, the primary particles do not growwell. In contrast, at heating temperatures over 1200° C. or a treatmenttime more than 10 hours, the primary particles may grow well but thecapacity of the resulting positive active material decreases at thefinal heat treatment.

The next step is to mix the resulting oxide compound with alithium-containing compound and then proceed through the first heattreatment. The first heat treatment is preferably performed for 5 to 20hours at 500 to 1000° C., and more preferably for 10 to 15 hours 700 to800° C. When the temperature is under 500° C. or the amount of time isunder 5 hours, the amount of carbon increases. In contrast, when thetemperature is over 1000° C. or the amount of time is over 20 hours, theparticles get too big and their functionality is deteriorated because itis hard to crush the particles.

The positive active material then proceeds through the second heattreatment, where heating is preferably performed for 10 to 40 hours at700 to 1000° C., or more preferably for 20 to 40 hours at 800 to 900° C.If the treatment temperature is under 700° C. or the time is under 10hours, the amount of the carbon present on the surface increases, whileif the temperature is over 1000° C. or the time is over 40 hours,workability is deteriorated because the material is hard to crush.

The hydroxide compound that is converted into the oxide compound isprepared by co-precipitating salts of nickel, cobalt, and optionallymanganese and a metal such as Al, Cr, Fe, Mg, Sr, V, and rare-earthelements. This preparation process is well-known and is well-describedin Korean Patent Application No. 2002-26200 in which a hydroxidecompound is prepared by mixing aqueous solutions containing sourcematerials and controlling the pH. Oxides, hydroxides, nitrates, andsulfates including nickel, cobalt, optionally manganese and a metal canbe used as the source materials. Lithium-containing compounds that reactwith the resulting oxide compound, include, but are not limited to,lithium hydroxide, lithium acetate, and lithium nitrate.

FIG. 1 shows a rechargeable lithium battery comprising the positiveactive material that embodies the present invention. The presentinvention provides a rechargeable lithium battery 1 comprising anegative electrode 2, a positive electrode 3, a separator 4 arrayedbetween the negative electrode 2 and the positive electrode 3, anelectrolyte to immerse the negative electrode 2, positive electrode 3,and separator 4, and a cylindrical battery container. The rechargeablelithium battery 1 is constructed by laminating the negative electrode 2,positive electrode 3, and separator 4, winding them in a spiral, andfinally placing those components into a battery container. However, theshape of a rechargeable lithium battery in the present invention is notlimited to the particular one shown in FIG. 1, and can be prismatic orpouch shaped.

The negative active material includes lithium metal or alithium-containing alloy, a material that is capable of reversiblyintercalating and deintercalating lithium ions, or a material that canform a lithium-containing compound by reversibly reacting with lithium.An exemplary material that is capable of reversibly intercalating anddeintercalating lithium ions is a carbonaceous material such ascrystalline carbon or amorphous carbon, or a carbon composite. Inaddition, representative exemplary materials to form alithium-containing compound that can reversibly react with lithium aretin oxide (SnO₂), titanium nitrate, silicon (Si), and so on, but it isnot limited thereto. The lithium-containing alloy is an alloy formed bycombining lithium and a metal including, but not limited to, Na, K, Rb,Cs, Fr, Be, Mg, Ca, Sr, Ba, Ra, Al, and Sn.

The electrolyte includes a lithium salt which is dissolved in anon-aqueous organic solvent. The lithium salt enables the rechargeablelithium battery to work by providing a source of lithium ions, andpromoting the movement of lithium ions between the positive and negativeelectrodes. Representative examples of the lithium salt include, morethan one or two from the group consisting of LiPF₆, LiBF₄, LiSbF₆,LiAsF₆, LiCF₃SO₃, LiN(CF₃SO₂)₃, Li(CF₃SO₂)₂N, LiC₄F₉SO₃, LiClO₄, LiAlO₄,LiAlCl₄, LiN(C_(m)F_(2m+1)SO₂)(C_(n)F_(2n+1)SO₂) (where m and n areintegers), LiCl, and LiI. The preferred concentration of the lithiumsalt ranges from 0.6 to 2.0 M. When the concentration of the lithiumsalt is under 0.6 M, the conductivity of the electrolyte decreases andthe performance of the electrolyte generally worsens. However, when theconcentration of lithium salt is over 2.0 M, the viscosity of theelectrolyte increases, which results in decreasing the movement oflithium ions.

The non-aqueous organic solvent acts as a medium through which ions canflow during the electrochemical reaction of a battery. The non-aqueousorganic solvent may include more than one compound such as a carbonate,ester, ether, or a ketone. As for the carbonate, a cyclic carbonate orlinear carbonate can be used. When more than one solvent is used, themixing ratio can be regulated according to the intended capacity of abattery by a person of ordinary skill in the art. Possible cycliccarbonates are ring carbonates which may include but are not limited to,ethylene carbonate, propylene carbonate, and a mixture thereof. Possiblelinear carbonates may include but are not limited to, dimethylcarbonate, diethyl carbonate, ethylmethyl carbonate, and methylpropylcarbonate. Possible esters may include but are not limited to,v-butyrolactone, valerolactone, decanolide, and mevaloratone and theketone may be, polymethylvinyl ketone, for example.

The following examples illustrate the present invention in furtherdetail. However, it is understood that the present invention is notlimited by these examples.

COMPARATIVE EXAMPLE 1

A positive active material of Li_(1.02)Ni_(0.8)Co_(0.1)Mn_(0.1)O₂ withan average primary particle diameter of 0.3 μm was prepared by mixingNi_(0.8)Co_(0.1)Mn_(0.1)(OH)₂ and LiOH in a mole ratio of Li:(Ni+Co+Mn)is 1.02:1, treating the mixture with heat for 10 hours at 700° C.,crushing it, and then performing a second heat treatment thereto for 20hours at 780° C.

COMPARATIVE EXAMPLE 2

Ni_(0.8)Co_(0.1)Mn_(0.1)O₂ was prepared by sinteringNi_(0.8)Co_(0.1)Mn_(0.1)(OH)₂ for 10 hours at 700° C. Next, a mixture ofthe Ni_(0.8)Co_(0.1)Mn_(0.1)O₂ and LiOH in a mole ratio of Li:(Ni+Co+Mn)of 1.02:1 was treated with heat for 10 hours at 800° C. It was thencrushed before performing a second heat treatment for 50 hours at 1050°C. The resulting compound is the positive active material ofLi_(1.02)Ni_(0.8)Co_(0.1)Mn_(0.1)O₂ with an average primary particlediameter of 5 μm.

EXAMPLE 1

Ni_(0.8)Co_(0.1)Mn_(0.1)O₂ was prepared by sinteringNi_(0.8)Co_(0.1)Mn_(0.1)(OH)₂ for 10 hours at 700° C. Next, a mixture ofthe Ni_(0.8)Co_(0.1)Mn_(0.1)O₂ and LiOH in a mole ratio of Li:(Ni+Co+Mn)of 0.94:1 was treated with heat for 10 hours at 700° C. It was thencrushed before performing a second heat treatment for 15 hours at 810°C. The resulting compound is the positive active material ofLi_(0.94)Ni_(0.8)Co_(0.1)Mn_(0.1)O₂ with an average primary particlediameter of 1 μm.

EXAMPLE 2

Ni_(0.8)Co_(0.1)Mn_(0.1)O₂ was prepared by firingNi_(0.8)Co_(0.1)Mn_(0.1)(OH)₂ for 10 hours at 700° C. Next, mixtures ofthe Ni_(0.8)Co_(0.1)Mn_(0.1)O₂ and LiOH in mole ratios of Li:(Ni+Co+Mn)of 0.94:1, and 0.96:1 were treated with heat for 10 hours at 700° C.,respectively. They were then crushed before performing a second heattreatment for 20 hours at 810° C. The resulting compounds are thepositive active materials of Li_(0.94)Ni_(0.8)Co_(0.1)Mn_(0.1)O₂, andLi_(0.96)Ni_(0.8)Co_(0.1)Mn_(0.1)O₂ with an average primary particlediameter of 2 μm.

EXAMPLE 3

Ni_(0.8)Co_(0.1)Mn_(0.1)O₂ was prepared by sinteringNi_(0.8)Co_(0.1)Mn_(0.1)(OH)₂ for 10 hours at 700° C. Next, mixtures ofthe Ni_(0.8)Co_(0.1)Mn_(0.1)O₂ and LiOH in mole ratios of Li:(Ni+Co+Mn)of 0.94:1, 0.96:1, and 0.98:1 were treated with heat for 10 hours at800° C., respectively. They were then crushed before performing a secondheat treatment for 20 hours at 810° C. The resulting compounds are thepositive active materials of Li_(0.94)Ni_(0.8)Co_(0.1)Mn_(0.1)O₂,Li_(0.96)Ni_(0.8)Co_(0.1)Mn_(0.1)O₂, andLi_(0.9)Ni_(0.8)Co_(0.1)Mn_(0.1)O₂ with an average primary particlediameter of 3 μm.

EXAMPLE 4

Ni_(0.8)Co_(0.1)Mn_(0.1)O₂ was prepared by firingNi_(0.8)Co_(0.1)Mn_(0.1)(OH)₂ for 10 hours at 700° C. Next, mixtures ofthe Ni_(0.8)Co_(0.1)Mn_(0.1)O₂ and LiOH in mole ratios of Li:(Ni+Co+Mn)of 0.94:1, 0.96:1, 0.98:1, and 1.00:1 were treated with heat for 10hours at 800° C., respectively. They were then crushed before performinga second heat treatment for 20 hours at 830° C. The resulting compoundsare the positive active materials ofLi_(0.94)Ni_(0.8)Co_(0.1)Mn_(0.1)O₂,Li_(0.96)Ni_(0.8)Co_(0.1)Mn_(0.1)O₂,Li_(0.98)Ni_(0.8)Co_(0.1)Mn_(0.1)O₂, and LiNi_(0.8)Co_(0.1)Mn_(0.1)O₂with an average primary particle diameter of 3 μm.

EXAMPLE 5

Ni_(0.8)Co_(0.1)Mn_(0.1)O₂ was prepared by firingNi_(0.8)Co_(0.1)Mn_(0.1)O₂ for 10 hours at 700° C. Next, mixtures of theNi_(0.8)Co_(0.1)Mn_(0.1)O₂ and LiOH in mole ratios of Li:(Ni+Co+Mn) of0.94:1, 0.96:1, 0.98:1, and 1.00:1 were treated with heat for 10 hoursat 800° C., respectively. They were then crushed before performing asecond heat treatment for 40 hours at 830° C. The resulting compoundsare the positive active materials of Li_(0.94)Ni_(0.8)Co_(0.1)Mn_(0.1)O₂Li_(0.96)Ni_(0.8)Co_(0.1)Mn_(0.1)O₂,Li_(0.98)Ni_(0.8)Co_(0.1)Mn_(0.1)O₂, and LiNi_(0.8)Co_(0.1)Mn_(0.1)O₂with an average primary particle diameter of 4 μm.

FIG. 2 shows the amount of carbon present on the surface of the positiveactive materials according to Comparative Example 1 and Examples 1 to 5,measured with a CS determinator (CS-444). The amount of carbon on thesurface of the positive active material of Examples 1 to 5, which embodythe present invention, is sharply lower compared to Comparative Example1.

FIGS. 3A to 3E show photographs of the primary particles of positiveactive materials taken with a scanning electron microscope (SEM)according to Comparative Example 1 and Examples 1 to 4, respectively.The average diameter of the primary particles in Examples 1 to 4 werebigger than those in the Comparative Example 1, as shown in FIGS. 3A to3E.

In order to coat the positive active material onto the positiveelectrode, a slurry was first formed. Slurries were made by combiningthe positive active materials prepared according to Comparative Examples1 and 2 and Examples 1 to 5, a conductive agent (super P), and a binder(PVDF) in a weight ratio of 94:3:3 with N-methyl pyrrolidone (NMP). Apositive electrode was fabricated by coating the slurry on an aluminumfoil, drying it, and compressing it with a roll press.

Similarly, a negative electrode was fabricated by coating a slurryconsisting of artificial graphite (PHS) for a negative active material,oxalic acid, and a binder (PVDF) in the weight ratio of 89.8:0.2:10 andNMP onto an aluminum foil. The foil was then dried and compressed with aroll press. Next, a separator made of a polyethylene (PE) porous filmwas inserted between the aforementioned positive and negative electrodesand placed in a battery container. Finally, an electrolyte solutionprepared by dissolving 1 mol/L LiPF₆ in a mixed solvent of propylenecarbonate (PC), diethyl carbonate (DEC), and ethylene carbonate (EC)(PC:DEC:EC=1:1:1) was injected into the battery container resulting in aprismatic rechargeable lithium battery.

The degree of swelling of the lithium battery fabricated according tothe aforementioned method was examined by checking the thickness beforeand after being charged to 4.2V at 0.5 C (Coulomb) and being left for 4hours in a chamber with a high temperature of over 85° C. Therechargeable lithium batteries comprising the positive active materialswith the a carbon fraction of less than 0.01 wt % of the total weight asin Examples 1 to 5 turned out to have excellent inhibition to swellingat a high temperature, showing only a small increase in thickness.

The discharge capacities of the rechargeable lithium batteries accordingto Examples 1 to 5 were measured, and the results are shown in FIG. 4.As shown in FIG. 4, the rechargeable lithium batteries had relativelyhigh discharge capacities. On the contrary, the rechargeable lithiumbattery including the positive active material of Comparative Example 1wherein the primary particles had an average diameter of 5 μm had a lowdischarge capacity.

A positive active material of the present invention has excellentinhibition to swelling at a high temperature as well as outstandingelectrochemical performance. The invention achieves this by reducing theamount of carbon on the surface and regulating the average diameter ofthe primary particles of the lithium-nickel-based positive activematerial.

The foregoing is considered illustrative only of the principles of theinvention. Further, since numerous modifications and changes willreadily become apparent to those skilled in the art, it is not desiredto limit the invention to the exact construction and operation shown anddescribed. Accordingly, all suitable modifications and equivalents fallwithin the scope of the invention and the appended claims.

1. A positive active material for a rechargeable lithium battery,comprising: a lithium/nickel-based compound; wherein primary particlesof said positive active material with an average particle diameterranging from 1 μm to 4 μm are agglomerated to form secondary particlesof the lithium/nickel-based compound.
 2. The positive active material ofclaim 1, wherein the average diameter of the secondary particles isbetween 50 μm to 20 μm.
 3. The positive active material of claim 1,wherein the amount of carbon on a surface thereof is less than 0.1% ofthe total weight of the positive active material.
 4. The positive activematerial of claim 3, wherein the amount of the carbon is between 0.05 wt% to 0.07 wt % of the total weight of the positive active material. 5.The positive active material of claim 4, wherein the amount of thecarbon is between 0.05 wt % to 0.06 wt % of the total weight of thepositive active material.
 6. The positive active material of claim 1,wherein the lithium/nickel-based compound has an X-ray diffractionintensity ratio I(003)/I(104) in X-ray diffraction pattern using CuKαranging from 1.04 to 1.15, and a full width at half maximum ranging from0.14 to 0.16.
 7. The positive active material of claim 1, wherein theactive positive material is represented by at least one of the followingformulas (1) to (5):Li_(x)Ni_(l-y)Co_(y)O_(2-z)A_(z)   (1)Li_(x)Ni_(l-y-z)Co_(y)M_(z)A₂   (2)Li_(x)Ni_(l-y-z)Co_(y)Mn_(z)O_(2-α)A_(α)  (3)Li_(x)Ni_(l-y-z)Co_(y)Mn_(z)O_(2-α)A_(α)  (4)Li_(x)Ni_(l-y-z)Co_(y)Mn_(z)M_(w)O_(2-α)A_(α)  (5) where x, y, z, w, anda are given by 0.94≦x≦1.1, 0≦y≦0.5, 0≦z≦0.5, 0≦w≦0.5, 0≦α≦2, wherein Mis at least one element selected from the group consisting of Al, Cr,Fe, Mg, Sr, V, and rare earth elements, wherein A is at least oneelement selected from the group consisting of O, F, S, and P.
 8. Amethod of preparing a positive active material, comprising: heating ahydroxide compound that includes nickel and cobalt, or nickel, cobalt,and at least one metal selected from the group consisting of Al, Cr, Fe,Mg, Sr, V, and rare earth elements, or nickel, cobalt, and manganese, ornickel, cobalt, manganese, and at least one metal selected from thegroup consisting of Al, Cr, Fe, Mg, Sr, V, and rare earth elements toobtain an oxide material; mixing the oxide material with alithium-containing compound; performing a first heat treatment to theresulting mixture; and performing a second heat treatment to the firstheat-treated mixture to obtain a lithium/nickel-based compound, whereinprimary particles of the positive active material have an averageparticle diameter ranging from 1 μm to 4 μm are agglomerated to formsecondary particles of the lithium/nickel-based compound.
 9. The methodof claim 8, wherein the average diameter of the secondary particles isbetween 5 μm to 20 μm.
 10. The method of claim 8, wherein the heattreatment of the hydroxide compound is performed at 500 to 1200° C. 11.The method of claim 8, wherein the first heat treatment is performed at500 to 1000° C.
 12. The method of claim 8, wherein the second heattreatment is performed at 700 to 1000° C.
 13. The method of claim 8,wherein the amount of carbon on the surface of the positive activematerial is less than 0.1 wt % of the total weight of the positiveactive material.
 14. The method of claim 13, wherein the amount of thecarbon is between 0.05 wt % to 0.07 wt % of the total weight of thepositive active material.
 15. The method of claim 14, wherein the amountof the carbon is between 0.05 wt % to 0.06 wt % of the total weight ofthe positive active material.
 16. The method of claim 8, wherein thelithium/nickel-based compound has an X-ray diffraction intensity ratioI(003)/I(104) in an X-ray diffraction pattern using CuKα ranging from1.04 to 1.15, and a full width at half maximum ranging from 0.14 to0.16.
 17. The method of claim 8, wherein the positive active material isrepresented by at least one of the following formulas (1) to (5):Li_(x)Ni_(l-y)Co_(y)O_(2-z)A_(z)   (1)Li_(x)Ni_(l-y-z)Co_(y)M_(z)A₂   (2)Li_(x)Ni_(l-y-z)Co_(y)Mn_(z)O_(2-α)A_(α)  (3)Li_(x)Ni_(l-y-z)Co_(y)Mn_(z)O_(2-α)A_(α)  (4)Li_(x)Ni_(l-y-z)Co_(y)Mn_(z)M_(w)O_(2-α)A_(α)  (5) where x, y, z, w, anda are given by 0.94≦x≦1.1, 0≦y≦0.5, 0≦z≦0.5, 0≦w≦0.5, 0≦α≦2, wherein Mis at least one element selected from the group consisting of Al, Cr,Fe, Mg, Sr, V, and rare earth elements, wherein A is at least oneelement selected from the group consisting of O, F, S, and P.
 18. Arechargeable lithium battery, comprising: a positive electrodecomprising a positive active material a negative electrode including anegative active material; and an electrolyte, wherein the positiveactive material is a lithium/nickel-based compound, wherein primaryparticles having an average particle diameter ranging from 1 μm to 4 μmare agglomerated to form secondary particles of the lithium/nickel-basedcompound; wherein the negative active material comprises a lithiummetal, a lithium-containing alloy, a material that is capable ofreversibly intercalating and deintercalating lithium ions, or a materialthat can form a lithium-containing compound by reversibly reacting withlithium
 19. The rechargeable lithium battery of claim 18, wherein thesecondary particles have an average diameter ranging from 5 μm to 20 μm.20. The rechargeable lithium battery of claim 18, wherein the negativeactive material is a carbonaceous material.
 21. The rechargeable lithiumbattery of claim 18, wherein an amount of carbon on a surface of thepositive active material is less than 0.1 wt % of the total weight ofthe positive active material.
 22. The rechargeable lithium battery ofclaim 21, wherein the amount of the carbon is between 0.05 wt % to 0.07wt % of the total weight of the positive active material.
 23. Therechargeable lithium battery of claim 21, wherein the amount of thecarbon is between 0.05 wt % to 0.06 wt % of the total weight of thepositive active material.
 24. The rechargeable lithium battery of claim18, wherein the lithium/nickel-based compound has an X-ray diffractionintensity ratio I(003)/I(104) in the X-ray diffraction pattern usingCuKα ranging from 1.04 to 1.15, and a full width at half maximum rangingfrom 0.14 to 0.16.
 25. The rechargeable lithium battery of claim 18,wherein the positive active material is represented by at least one ofthe following formulas (1) to (5):Li_(x)Ni_(l-y)Co_(y)O_(2-z)A_(z)   (1)Li_(x)Ni_(l-y-z)Co_(y)M_(z)A₂   (2)Li_(x)Ni_(l-y-z)Co_(y)Mn_(z)O_(2-α)A_(α)  (3)Li_(x)Ni_(l-y-z)Co_(y)Mn_(z)O_(2-α)A_(α)  (4)Li_(x)Ni_(l-y-z)Co_(y)Mn_(z)M_(w)O_(2-α)A_(α)  (5) where x, y, z, w, anda are given by 0.94≦x≦1.1, 0≦y≦0.5, 0≦z≦0.5, 0≦w≦0.5, 0≦α≦2, wherein Mis at least one element selected from the group consisting of Al, Cr,Fe, Mg, Sr, V, and rare earth elements, wherein A is at least oneelement selected from the group consisting of O, F, S, and P.